In an interview on The Diane Rehm Show, Kenneth Slawenski, author of the new biography J. D. Salinger: A Life, claims that he doesn't know what this line means. In the biography, he calls the ending of Catcher "ambiguous" and says that "Salinger has deliberately left it to readers to insert their own selves, their own doubts, aspirations, and dissatisfactions, in order to complete [Holden's] journey."
I've never felt any ambiguity in that line or the ending of The Catcher in the Rye. To me, it expresses the essence of the spiritual battle that Salinger and anyone who commits to self-awareness/God consciousness/Oneness (insert whatever phrase suits you) must face. If you connect—to yourself and to others—if you really share beauty, God, your essence, you will have acknowledged that everyone is God . . . and, so long as you can tolerate feeling this epiphany, you will feel a sweet missing. You will simultaneously hurt and feel unbearable sweetness. I believe this sweet missing is a God state. God misses God: Because God is lonely to see God, we fractured little God beings play this game to relearn who we are. Loneliness drives us to know our God selves. And when we do, we feel the sweet missing at the root of the craving. Or, as Holden says, "you start missing everybody." (more…)